High temperature superconducting materials have been extensively developed over the last few years. They can now be manufactured for use in various devices including cables, transformers, motors and MAGLEV transport vehicles. A Bi-2223 high temperature superconducting (HTS) composite, using Bi-2223 high temperature superconducting material, has become one of the most promising HTS materials as a result of good producibility, and its high electrical and mechanical properties. Currently, the most widely used method for fabricating high temperature superconduting composites is the powder-in-tube (PIT) technique. A composite consists of superconducting filaments surrounded by a matrix of silver and/or silver alloy, to form a multi-filamentary article. Strain introduced in the conducting portion of high temperature superconducting composites during processing, handling and service may result in micro-cracks in the superconducting grains and even fractures in some of the filaments. Such cracks and fractures may reduce the available superconducting cross-sectional areas that can transport current. In this case, the overall electrical current load will be shared by the remaining filaments. If the current exceeds that which those remaining filaments can carry, the excess current will be transferred into the matrix. Usually the matrix materials are silver and/or silver alloy, which can not withstand large currents, such as 100A, so that the matrix will melt as a result of resistive heating resulting in a so called “burn-out”. Consequentially, the high temperature superconducting composites may lapse into a non-superconducting state and the whole superconducting device may fail. This problem may bring difficulties in the practical applications of high temperature superconducting composites. In summary, a special method is necessary to protect the electrical properties of high temperature superconducting composites. By such a method when there are cracks and fractures within the filaments, current may flow through other paths to prevent the burn-out of the composites, hence maintaining the stable operation of superconducting devices.
Up to now, several structural arrangements of high temperature superconducting composites have been provided. A brief summary of some of the advantages and disadvantages of these layouts is given in the following.
(1) Mono-Filamentary Composites
A structure is described by Jan Boeke in U.S. Pat. No. 5,006,671 as shown in FIG. 1, where 1 is a superconductor, 2 is a glass-clad wire. Higher critical current density values have been obtained on superconductor wires having this kind of simple geometry. Moreover, owing to their high filling factor (FF), in the range 0.4<FF<0.5, where FF=[Bi-2223]/([Bi-2223]+[Ag]), large critical current density values correspond also to high engineering current density values. Another form of mono-filamentary composites is the Bi-2223/Ag/barrier/Ag structure provided by P. Kovac et al. (Supercond. Sci. Technol., vol. 13, pp378-384, 2000) shown in FIG. 2, where 1 is a Bi-2223 superconductor, 3 is a barrier layer, and 4 is a silver layer. These composites show reduced ac losses.
Although a mono-filamentary structure has some advantages, it has only a single superconducting region. If the conducting portion of a mono-filamentary high temperature superconducting composite fractures, the overall current will be carried by the silver and silver alloy matrixes surrounded the fracture. The matrix might not withstand the large current, resulting in burn-out. Consequentially, the high temperature superconducting composite may lapse into a non-superconducting state, severely degrading the stability of any superconducting device into which the composite is incorporated.
(2) Multi-Filamentary Composites
A structure investigated by Mukai et al. in U.S. Pat. No. 5,869,430 and Lin Yubao et al. (Chinese journal of low temperature physics, vol.21, No. 2, pp122-130, 1999) is shown in FIG. 3, where 1 is a superconductor, 5 is a noble metal or a noble metal alloy, 6 is a tube aperture into which a superconductor material may be added, and 7 is a silver or silver alloy tube. A high temperature superconducting tape can be made by rolling the above tube, see FIG. 4, where 1 is a superconductor, and 7 is a silver or silver alloy matrix. In a multi-filamentary tape a matrix of noble metal or noble metal alloy encloses each superconducting filament that has a ribbon shape and is substantially uniformly distributed in the cross section of the tape. Compared to mono-filamentary composites, multi-filamentary composites exhibit better mechanical strength, with a higher critical bending strain of >0.3%. However, they have low critical current density values. Another kind of superconducting tape that has a different structure is provided by N. V. Vo et al. (Journal of Magnetic Materials, vol. 188, pp145-152, 1998). The conductor used for coil winding is reduced gradually in size (cross-sectional area) with the outward radial coil distance according to its Ic versus B characteristic curve. This method makes use of the fact that since the magnetic field drops off radially from the center of the coil, thinner wires or tapes can be utilized to carry the same critical current. This method minimizes the use of material, and is cost effective. P. Kov et al. (Supercond. Sci. Technol., vol.13, pp378-384, 2000) also reported a multi-filamentary Bi-2223/Ag/barrier/Ag composite structure, which exhibits similar properties to mono-filamentary composites. These kinds of composites are however of low critical current density and not suited for extensive industrial use.
(3) Concentric Composites
A structure is provided by L. Martini et al. (Supercond. Sci. Technol., vol.11, pp231-237, 1998) shown in FIGS. 5 and 6, where 1 is a superconductor and 7 is a silver or silver alloy sheath. Superconductors are separated by coaxial silver or silver alloy tubes. In FIGS. 5 and 6, one and three superconductor rings are shown respectively. Concentric composites exhibit better electrical and bending properties, similar to those of multi-filamentary tapes (Jc>40 kAcm−2, γc>0.3%). The silver or silver alloy matrix enhances the thermostability of these composites, and can provide an alternative path for the current, in case of a HTS transition to the normal conducting state. The disadvantage of these composites is the increased ac losses.
(4) Multi-Layered Composites
A structure is provided by L. Martini et al. (Supercond. Sci. Technol., vol.7, pp24-29, 1994) shown in FIG. 7, where 8 is a silver foil, and 1 is a superconducting layer. A pure silver foil, 80-100 μm thick, is folded. The folded silver substrate is then filled with the superconducting powder and the metal-ceramic composite is pressed and heat-treated to form multi-layered tapes with superconducting layers separated by metal layers. Multi-layered composites exhibit a high value of FF, and large critical transport currents, up to Ic=300A at 77 K. However, these tapes can only be made in short length of less than 1 m, hence limiting their practical applications.
Based on the above structures and characteristics of high temperature superconducting materials, the following requirements should be met for the structure of high temperature superconducting composites.
(1) Fine
It is usually necessary to bend superconducting composites for practical applications in high temperature superconducting devices. When the composites are bent, it is required that no significant reduction of their critical current density values occurs. The bending strain is equal to h/D according to deformation theory (Z. Han et al., MT-15, Beijing, October, pp20-24, 1997), where h and D are the thickness and bending diameter of a superconducting composite respectively. A critical bending strain, which has been suggested to be about 0.2%, is usually defined as the strain that results in 5% degradation of its critical current. If the bending strain exceeds the critical bending strain value of the material, the current carrying capability will be significantly reduced. A thinner superconducting composite will therefore lead to smaller bending strain under the condition of constant D.
(2) Multi-Filamentary
If one filament, or some filaments, within a multi-filamentary composite fractures, there are other alternative current paths in the composite. Moreover, multi-filamentary composites exhibit better mechanical properties. Typically, the development of performance affecting cracks in response to bending strains is more likely to occur in mono-filamentary composites than in multi-filamentary composites, where the critical bending strain values increase with the number of filaments in the composites, and can be greater than 1.0%. Other limitations of mono-filamentary composites include a poorer crack healing ability and increased oxygen access to the superconductor during processing.
(3) Tape-Shaped
Taped-shaped composites with a thinner thickness may have smaller bending diameters compared to cylindrical composites, and therefore are easier to manufacture into superconducting devices such as coils.
(4) Parallel Connection of Several Composites
When current flows in a filament, it is only carried by the superconductor. When fracture occurs in one or more filaments, the total electrical current will flow through the remaining filaments. If the current exceeds that which those remaining filaments can carry, the excess current will be transferred into the matrix, which might not withstand the large current, resulting in burn-out of the matrix. Consequentially, the high temperature superconducting composite may lapse into a non-superconducting state and the superconducting device into which the composite is incorporated may fail. In the case of parallel connection of several composites, however, the filaments in other composites may provide alternative current paths.
In summary, high temperature superconducting material may be used to fabricate thin multi-filamentary composites with parallel connection among several composites in order to protect against reduction of superconductivity.